The invention provides new types of titanium/zirconium catalysts, a process for preparing these catalyst, and their use in esterification reactions, transesterification reactions and polycondensation reactions to prepare esters or polyesters.
Polyesters are important industrial polymers which have been used for some time now, for example, to produce fibers or as polyol components in polyurethane systems (see G. Oertel in Polyurethane Handbook, Hanser Publishers (1994), p. 65-72). Polyesters are prepared either by direct esterification of low molecular weight polycarboxylic acids (e.g. adipic acid) with low molecular weight polyalcohols (e.g. diethylene glycol) or by transesterification starting from alkyl esters of polycarboxylic acids and polyalcohols. A catalyst is generally used in either of the reactions mentioned above in order to shorten the reactor occupation times and to build up high molecular weights (see R. E. Wilfong in Journal of Polymer Science, vol. 54 (1961), p. 385-410 and A. Fradet, E. Marxc3xa9chal in Advances in Polymer Science 43, Springer Verlag (1982), p. 51-142). Many commercial processes use, for example, manganese, zinc, antimony or tin salts as catalysts for esterification, transesterification or polycondensation reactions. However, the problem with some of the metal compounds mentioned above is the tendency to form insoluble complexes which can cause problems during further processing of the esters or polyesters. In addition, the physiological unacceptability and potentially harmful effects in the environment of the classes of compounds mentioned above are increasingly acting against their use as catalysts.
Organic titanium compounds, in particular titanium orthoesters such as, for example titanium tetraisopropylate and titanium tetra-n-butylate, are also effective and frequently used esterification, transesterification and polycondensation catalysts. See, for example, U.S. Pat. No. 2,822,348. General use of these titanium catalysts, however, is restricted due to a number of disadvantages. Thus, organic titanium compounds can be insoluble in the reaction mixture (e.g. poly(titanium alkylate)), which leads to uneven distribution of the catalyst and has an effect on its activity. Furthermore, certain titanium catalysts are hydrolysis-sensitive which impairs the storage-stability of the catalyst systems. In addition, polymeric titanium compounds may be produced during the course of reaction, which ultimately results in a cloudy reaction product which can be purified only by means of a costly filtration procedure. In addition, titanium compounds normally react with traces of impurities such as, for example, aldehydes, which are also produced during the esterification, transesterification or polycondensation reaction and in this way produce unwanted yellow coloration in the reaction product. It has been shown that such unwanted discoloration can be counteracted by the use of multi-component catalyst systems. Thus, for example, U.S. Pat. No. 6,080,834 describes a catalyst system consisting of a titanium compound, a complexing reagent (e.g. a hydroxycarboxylic acid), a phosphorus compound and, optionally, other additives in a solvent. The use of catalyst systems consisting of titanium orthoesters, alcohols (e.g. ethylene glycol), 2-hydroxycarboxylic acids and bases (e.g. sodium hydroxide) is described in, for example, U.S. Pat. No. 5,866,710. The preparation of such complicated catalyst systems is obviously costly and therefore has a large economic impact when used in esterification, transesterification and polycondensation reactions. In addition, there is the risk that further processing of the esters or polyesters in various applications (e.g. polyurethane systems) may be impaired by the presence of the additives mentioned above in the catalyst systems.
For the reasons discussed above, there is a need for new types of catalysts which are characterized by the simplest possible composition, high activity in esterification, transesterification and/or polycondensation reactions, adequate solubility in the starting components and in the end product, and good resistance to hydrolysis, and which are physiologically acceptable. It is also desirable that catalysts according to the invention lead to a reaction product with improved optical properties (e.g. less unwanted color in the polyester).
Surprisingly, it has now been found that when using suitable starting materials new types of catalysts can be prepared which, in comparison to the catalysts described above, are characterized by a simpler, and thus, more economically viable composition and which exhibit good resistance to hydrolysis. In addition, catalysts according to the invention provide a reaction product with improved optical properties, i.e. less unwanted color in the esters or polyesters. Other advantages of catalysts according to the invention (e.g. high activity, good solubility in the starting components and in the end product, physiological acceptability) are also obtained.
The present invention thus provides titanium and/or zirconium catalysts comprising the reaction product of:
a) one or more orthoesters or one or more condensed orthoesters of titanium and/or zirconium; with
b) one or more polyalcohols which contain at least two hydroxyl groups and have a number average molecular weight of at least 180 g/mol;
wherein the molar ratio of polyalcohol b) to orthoester or condensed orthoester of titanium and/or zirconium a) is at least 2:1.
Orthoesters or condensed orthoesters of titanium or zirconium a) are used to prepare catalysts according to the invention. In a preferred embodiment, the orthoesters correspond to the general formula:
M(OR)4, 
wherein:
M: represents titanium or zirconium, and
each R: may be the same or different, and individually represents a linear alkyl group, a branched alkyl group or a cyclic alkyl group.
Suitable alkyl groups R in the general formula above preferably contain from 1 to 6 carbon atoms.
Particularly preferred orthoesters are titanium tetraisopropoxylate and titanium tetra-n-butoxylate.
Condensed orthoesters for the preparation of the catalysts of the invention are typically obtained by careful, partial hydrolysis of the titanium/zirconium orthoester and, inter alia but not exclusively, are represented by the general formula:
R1O[M(OR1)2O]nR1, 
wherein:
M: represents titanium or zirconium; and
each R1: may be the same or different, and individually represents a linear alkyl group, a branched alkyl group or a cyclic alkyl group;
n: represents a number less than 20, and preferably a number less than 10.
In the formula above, the group R1 preferably contains 1 to 6 carbon atoms.
Particularly preferred condensed orthoesters are poly(titanium isopropoxylate) and poly(titanium butoxylate).
The process for the preparation of the titanium and/or zirconium catalysts of the present invention comprises reacting
a) one or more orthoesters or one or more condensed orthoesters, with
b) one or more polyalcohols. Suitable polyalcohols for the present invention include those polyalcohols which contain at least two hydroxyl groups and have a number average molecular weight of at least 180 g/mol, in particularly preferred are those polyalcohols with two to six hydroxyl groups and a molecular weight of 180 g/mol to 4500 g/mol.
In a preferred embodiment, polyalcohols b) are polyetherpolyols. Polyetherpolyols used according to the invention may be prepared, for example, by polyaddition of alkylene oxides to polyfunctional starter compounds in the presence of caesium, rubidium, strontium or barium hydroxide or alternative basic catalysts. Polyetherpolyols used according to the invention are preferably prepared using a highly active double metal cyanide catalyst from a starter compound with on average 2 to 8, preferably 2 to 6, active hydrogen atoms and one or more alkylene oxides, as is described, for example, in U.S. Pat. No. 5,545,601 (believed to correspond to EP-A 761,708), the disclosure of which is herein incorporated by reference.
Preferred starter compounds for the polyetherpolyols include, for example, compounds with at least two hydroxyl groups per molecule such as 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, glycerine, trimethylolpropane, pentaerythritol, sorbitol and saccharose. Further preferred starter compounds include ammonia or compounds which contain at least one primary or secondary amine group such as, for example, aliphatic amines such as 1,2-diaminoethane, oligomers of 1,2-diaminoethane (for example diethylenetriamine, triethylenetetramine or pentaethylenehexamine), 1,3-diaminopropane, 1,3-diaminobutane, 1,4-diaminobutane, 1,2-diaminohexane, 1,3-diaminohexane, 1,4-diaminohexane, 1,5-diaminohexane, 1,6-diaminohexane, aromatic amines such as 1,2-diaminohexane, 1,3-diaminobenzene, 1,4-diaminobenzene, 2,3-diaminotoluene, 2,4-diaminotoluene, 3,4-diaminotoluene, 2,5-diaminotoluene, 2,6-diaminotoluene, 2,2xe2x80x2-diaminodiphenylmethane, 2,4xe2x80x2-diaminodiphenylmethane, 4,4xe2x80x2-diamino-diphenylmethane or other aromatic amines which are obtained by acid-catalyzed condensation of aniline with formaldehyde or compounds which contain a hydroxyl group in addition to a primary, secondary or tertiary amine group such as, for example, ethanolamine, diethanolamine or triethanolamine. The starter compounds may be used individually or as a mixture.
Starter compounds which are particularly preferred for reaction with a highly active double metal cyanide catalyst can be obtained, for example, by conventional alkali catalysis from the previously mentioned hydroxy-functional low molecular weight starter compounds and alkylene oxides such as oxirane, methyloxirane and/or ethyloxirane.
Preferred alkylene oxides for preparing polyetherpolyols for use as polyalcohols in accordance with the invention are oxirane, methyloxirane and ethyloxirane. These may be used either individually or as a mixture. When used in a mixture, it is possible to incorporate the alkylene oxides in a random or blockwise manner or to use the two processes one after the other.
One or more polyesterpolyols consisting of aromatic and/or aliphatic polycarboxylic acids and polyols which contain at least two hydroxyl groups may also be used, either exclusively or in a mixture with the polyetherpolyols described above, as polyalcohol component b).
Examples of suitable dicarboxylic acids are phthalic acid, isophthalic acid, terephthalic acid, fumaric acid, maleic acid, azelaic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, malonic acid and succinic acid. The pure dicarboxylic acids or any mixtures thereof at all may be used. Instead of the free dicarboxylic acids, the corresponding dicarboxylic acid derivatives such as, e.g. monocarboxylates or dicarboxylates of alcohols with one to four carbon atoms may also be used. Dicarboxylic anhydrides such as phthalic anhydride or maleic anhydride may be used as carboxylic acid components.
The following are preferably used as polyol components for esterification: ethylene glycol, diethylene glycol, tetraethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,10-decanediol, glycerine, trimethylolpropane, pentaerythritol or mixtures thereof.
Polyesterpolyols prepared from lactones, e.g. xcex5-caprolactone, or hydroxycarboxylic acids, e.g. xcfx89-hydroxycarboxylic acids, may also be used. The polyalcohol may also contain polyetheresterpolyols such as can be obtained, e.g. by reacting phthalic anhydride with diethylene glycol followed by reaction with oxirane.
The polyalcohol component b) may also contain, in addition to the polyol (e.g. polyether, polyester) having an average OH-functionality of at least 2.0, up to an amount of 80 wt. % of one or more monofunctional alcohols.
The catalysts according to the invention are prepared by reacting the orthoester or condensed orthoester of titanium or zirconium a), with one or more polyalcohols b), wherein the molar ratio of polyalcohol to titanium and/or zirconium compound, as mentioned, is at least 2:1. It is preferred to use 3 to 5 moles of polyalcohol per mole of titanium and/or zirconium compound. The catalyst according to the invention is prepared by mixing components a) and b) and removing any secondary products which have formed (such as 1-butanol, when the orthoester is, for example, titanium tetra-n-butylate). In a preferred embodiment, the polyalcohol is initially introduced, the orthoester or condensed orthoester is added, and then the secondary products which form are distilled off at temperatures of from 80xc2x0 C. to 250xc2x0 C., preferably from 100xc2x0 C. to 200xc2x0 C., optionally under reduced pressure. The catalyst according to the invention can be dissolved in all common solvents such as alcohols or glycols.
Preferred solvents include compounds such as, for example, ethylene glycol, diethylene glycol, dipropylene glycol, butanediol and/or hexanediol.
The present invention also provides a process for preparing titanium and/or zirconium catalysts according to the invention comprising reacting
a) one or more orthoesters or one or more condensed orthoesters of titanium and/or zirconium; with
b) one or more polyalcohols which contain at least two hydroxyl groups and have a number average molecular weight of at least 180 g/mol;
wherein the molar ratio of polyalcohol b) to orthoester or condensed orthoester of titanium and/or zirconium a) is at least 2:1.
Furthermore, the invention also provides a process for the preparation of (poly)esters (i.e. esters and/or polyesters) by appropriate esterification, transesterification or polycondensation reactions in the presence of the novel titanium and/or zirconium catalysts of the present invention.
Esters or polyesters can be prepared, as mentioned above, by direct esterification of low molecular weight polycarboxylic acids or carboxylic anhydrides with low molecular weight polyalcohols in the presence of the novel catalysts of the present invention, or by a transesterification reaction starting from alkyl polycarboxylates and polyalcohols in the presence of the novel catalysts of the present invention. Direct esterification and transesterification may also be performed starting from hydroxycarboxylic acids. A preferred process for preparing esters or polyesters in the presence of the catalysts according to the invention is performed by means of a polycondensation reaction.
Examples of low molecular weight polycarboxylic acids for preparing esters and/or polyesters include compounds such as phthalic acid, isophthalic acid, terephthalic acid, fumaric acid, maleic acid, azelaic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, malonic acid and succinic acid. The pure dicarboxylic acids or any mixtures thereof may be used. Dicarboxylic anhydrides such as, for example, phthalic anhydride or maleic anhydride may also be used. Instead of the free polycarboxylic acids or carboxylic anhydrides, appropriate dicarboxylic acid derivatives such as, for example, monocarboxylates or dicarboxylates of alcohols with one to four carbon atoms may also be used. Lower homologues such as, for example, methyl esters are normally used because the alcohol being produced during reaction is then removed by distillation.
The following are preferably used as polyalcohol components for preparing esters or polyesters: ethylene glycol, diethylene glycol, tetraethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,10-decanediol, glycerine, trimethylolpropane pentaerythritol or mixtures thereof.
Preparation according to the invention of esters or polyesters by esterification, transesterification or polycondensation in the presence of the catalysts according to the invention may be performed over a wide range of reaction conditions. Reaction is preferably performed in a temperature interval from 100xc2x0 C. to 350xc2x0 C., particularly preferably 150xc2x0 C. to 230xc2x0 C., wherein the pressure is varied between 0.001 bar and atmospheric pressure, depending on progress of the reaction. The amount of catalyst added during the preparation process depends on the titanium or zirconium content (Ti/Zr) of the catalyst. Normally, the metal concentration is in the range 0.1 ppm to 500 ppm, preferably 1 ppm to 12 ppm, with respect to the entire mass of the reaction product.
Preparation of a polyesterpolyol using adipic acid and a mixture of diethylene glycol and trimethylolpropane is preferred. In the case of a typically batchwise procedure, the starting materials are heated to a temperature of up to 200xc2x0 C. in the reactor and the water being produced is distilled off under atmospheric pressure. After adding the catalyst according to the invention (1 to 12 ppm of metal), the pressure is reduced stepwise to 15 mbar. The water of reaction is distilled off at 200xc2x0 C. until the acid value is xe2x89xa61.0 mg KOH/g. Obviously, the polyester may be worked up in a short-path evaporator (200xc2x0 C./0.05 mbar) to remove cyclic ester fractions.
Polyesters according to the invention may be used to prepare polyurethane foams, in particular flexible foams. In addition, polyesters according to the invention may obviously also be used to prepare other polyurethane (PUR) systems (e.g. cast elastomers, fibers).
The process according to the invention represents a new, effective method of preparation of esters or polyesters which are characterized in particular by excellent color quality.
The contents of the present invention are intended to be explained with selected examples.